In video compression technology, interframe coding (e.g. motion predictive coding) is most often used to remove temporal redundancy in the video sequence. For example, the most popular video coding standards, including H.261, MPEG-1, MPEG-2, H.263, and the new MPEG4 standard, all use motion predictive coding to remove temporal redundancy. In order to facilitate interframe decoding, frame memory is usually required to store a reference frame(s) (e.g., a previous frame). The cost of this frame memory can be a very significant part of a digital video decoder, especially for higher-resolution video. By far, the memory cost predominates in the cost of an HDTV MPEG-2 decoder.
Many researchers have attempted to reduce the memory requirements for reference frame storage. Generally, these methods require that the reference frames be compressed prior to storage in the decoder frame memory and then decompressed when they are needed to perform interframe decoding. Thus the common elements of systems implementing these methods are an image compressor, a compressed image storage memory, and an image decompressor.
The earliest such systems simply downsampled the reference frames prior to storage. For instance, U.S. Pat. No. 5,614,952 proposes that reference frames be either subsampled at a lower resolution, or that pixel values be truncated to a lower precision. U.S. Pat. No. 5,825,424 implements this method in a configurable processor that can be used in different configurations with different amounts of memory. Systems such as these are relatively low in complexity, but tend to suffer greatly from drift. Drift typically manifests itself as visible temporal cycling in reconstructed picture quality, due to the errors introduced in interframe decoding by throwing away part of the reference information during downsampling.
Later systems added complexity in an attempt to combat drift. U.S. Pat. No. 5,777,677 proposes Hadamard transforming a reference frame and quantizing the transform to achieve compression. European Patent Application 0,794,673 A2 proposes multiplying image blocks by a one-dimensional "Harr" transform matrix and quantizing the product to achieve compression. U.S. Pat. No. 5,680,129 codes pixel values directly with a predictive coder. European Patent Application 0,778,709 A1 codes pixel values directly with an adaptive pulse-code modulation technique. And European Patent Application 0,687,111 A2 compresses reference frames using an MPEG-like approach with a DCT, quantizer, and variable-length coder.